The I-400 class was the Japanese submarine class that carried aircraft and became the largest submarine type built during World War II.
Japan designed the I-400 class to attack targets far from home waters. Each boat carried up to three specially designed Seiran floatplanes in a watertight hangar. The aircraft could be launched from a forward catapult after the submarine surfaced, although they could not land back on the submarine and were expected to ditch or reach a friendly base.
The boats displaced more than 6,000 tonnes submerged and were unusually long, giving them the range needed for intercontinental missions. Japan planned to use them against the Panama Canal, but the war ended before that operation took place. The submarines were later captured by the United States and scuttled in 1946.
The I-400 class is often called the largest non-nuclear submarine class of its era. It was surpassed in size by later ballistic-missile submarines.